1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus for transmitting and receiving ultrasonic waves to perform imaging of organs, bones, etc. within a living body thereby generating ultrasonic images to be used for diagnosis.
2. Description of a Related Art
In an ultrasonic imaging apparatus to be used for medical diagnoses, an ultrasonic probe including plural ultrasonic transducers having transmitting and receiving functions of ultrasonic waves is used. When an ultrasonic beam formed by synthesizing plural ultrasonic waves is transmitted from such an ultrasonic probe to an object to be inspected, the ultrasonic beam is reflected at a boundary between regions having different acoustic impedances, i.e., between tissues within the object. Thus generated ultrasonic echoes are received and an image is constructed based on the intensity of the ultrasonic echoes, and thereby, the state within the object can be reproduced on a screen.
As described above, a general ultrasonic image represents a shape of a tissue within the object by utilizing the difference in acoustic impedance between tissues. Accordingly, in an ultrasonic image of an organ like a liver which is formed by a number of reflectors nearly in size of the wavelength of ultrasonic waves, speckle pattern components, in which light points and dark points are scattered, mainly appear due to interference between ultrasonic echoes. In such an ultrasonic image, even when a tumor or the like is included in a tissue within the organ, no distinct reflection surface is seen at the outline of the tissue, and therefore, the difference between a normal tissue and an abnormal tissue can be determined only by the difference between speckle patterns. Accordingly, determination of tissue property of the tumor or the like and medical diagnoses based thereon are difficult. Further, with respect to a region like the vicinity of the bone part where soft tissues such as muscles and hard tissues such as bones, tendons, and nucleus pulposus are intricate, it is also extremely difficult to visually recognize the soft tissues from the hard tissues. Therefore, when an ultrasonic image is generated, the use of elements other than intensity of ultrasonic echoes has been studied. As the elements, it is conceivable to utilize statistical property (statistics values) representing interrelationships among plural ultrasonic echo signals respectively received by plural ultrasonic transducers.
As a related technology, International Publication WO00/40997 discloses that the obtained echo signals are processed along both processing paths of one reception signal processing path using time delays set for a traditional coherent receive beam forming and another reception signal processing path using time delays set to apply incoherent summing using time delays equal to, for example, zero and an ultrasonic image is generated based on thus obtained coherent summation signals and incoherent summation signals in order to prevent incoherent summation of phase matching signals due to variations in propagation times and image deterioration in an ultrasonic image by suppressing a display based on incoherent summation signals (page 1). Further, in WO00/40997, an image is generated based on a coherence factor, and displayed as a color map overlaid on a B-mode image. Here, the coherence factor refers to the degree of similarity of a signal that has been phase matched (coherent summed signal A) and a signal that has not been phase matched (incoherent summed signal B), and expressed by the difference between the signal A and signal B, the ratio of the signal A to the signal B, or the like.
According to WO00/40997, it can be expected that the image quality of an ultrasonic image may be improved by making a choice among reception signals based on the coherence factor. However, a tissue property or the angle relative to an ultrasonic beam of a reflector is not obtained. Further, any analysis of signals before phase matching (i.e., plural reception signals that form the above-mentioned signal A or signal B) is not performed.
Further, Japanese Patent Application Publication JP-A-11-235341 discloses an ultrasonic diagnostic apparatus for providing directional characteristics of transmission and reception by providing individual delay times to excitation signals of respective arranged plural vibrators and reception signals obtained by these vibrators by receiving ultrasonic reflection waves from an object to be inspected and obtaining an ultrasonic image by scanning within the object with the ultrasonic waves provided with directional characteristics in order to suppress influence of the distortion on image quality even when waveforms of reception signals are distorted due to refraction, multiple reflection, or the like (page 1). The ultrasonic diagnostic apparatus includes a reception signal evaluation unit for evaluating the distortion of the reception signals with respect to each vibrator and an aperture control unit for controlling at least one of intensity of the excitation signals and amplification factor of the reception signals based on a result of the evaluation, wherein the degree of distortion is evaluated by utilizing waveform similarity, correlation coefficients, intensity, or the like of the plural reception signals.
According to JP-A-11-235341, it can be expected that the image quality of an ultrasonic image may be improved by using the waveform distortion of the reception signals as a parameter. However, also in JP-A-11-235341, a tissue property or the angle relative of a reflector to an ultrasonic beam is not obtained. Further, the intensity of reception signals is used for waveform similarity, but various kinds of statistical property are not utilized.
JP-A-10-258052 discloses a wave receiving apparatus including a receiver for detecting orientation or displacement of a target of detection by one reception with a predetermined aperture in an arbitrary position and receiving wave that has reached within the aperture with information on a position within the aperture in order to obtain high resolving power or obtain signals equivalent of reception signals corresponding to an aperture that actually receives no signal, a weighting processing unit for weighting the signals obtained by the receiver by using respective kinds of weighting functions having variables corresponding to the position within the aperture, and a computing unit for performing a computation including a computation for obtaining a propagation direction of the wave that has reached the aperture or a position of a wave source that has generated the wave based on plural weighted reception signals obtained by weighting processing by the weighting processing unit (page 1) .
According to JP-A-10-258052, the orientation and position of the target of detection can be detected by weighting the reception signals according to the position within the aperture. However, mutual property and statistics values of reception signals obtained from plural ultrasonic transducers are not utilized, and a tissue property of a target object cannot be identified.
JP-A-8-117225 discloses a living tissue evaluation apparatus including transmitting means for transmitting ultrasonic waves to a living tissue, intensity distribution obtaining means for obtaining an intensity distribution of ultrasonic waves by receiving ultrasonic waves that have been transmitted through the living tissue and spread, and evaluation value computing means for calculating an evaluation value of the living tissue based on the obtained intensity distribution for analyzing a microscopic structure of the living body by utilizing information on spatial spreading of ultrasonic waves transmitted through the living tissue (page 1).
According to JP-A-8-117225, the intensity distribution among ultrasonic vibrators is obtained. However, since an interference phenomenon in transmission is used, information on the depth by the ultrasonic beam cannot be obtained and property within the tissue is obtained only as integration information. Further, any information within objects can be obtained except for an object within which ultrasonic interference occurs.
Further, JP-P2003-61964A discloses an ultrasonic diagnostic apparatus for applying ultrasonic pulses to an object to be inspected to obtain a tomographic image, smoothing the image by utilizing statistical property of a speckle pattern, and extracting a microstructure in order to observe a minute abnormal lesion within a homogeneous tissue structure (page 2). The ultrasonic diagnostic apparatus includes analysis computation means for extracting a specific signal by using intensity or statistical property of amplitude information of echo signals generated from a part of the object, and display means for displaying a result extracted from the analysis computation means.
According to JP-P2003-61964A, the intensity statistics value of a signal after phase matching is imaged, however, any element-level signal is not referred to.
Thus, according to the above documents, the image quality improvement in ultrasonic image may be expected, however, tissue property of a reflector cannot be obtained.